Processing of green tea leaf to produce a variety of tea products is well known. In a typical case, freshly picked green tea leaves are allowed to wither involving mild drying for up to 24 hours, and the withered leaves are then cut up in a maceration step in which the leaves are passed through sets of CTC (crush, tear, curl) rollers that shred the leaves. The shredded leaves are subjected to a fermentation step for about 2 hours, during which enzymic oxidation takes place and the leaves change colour from green to brown as a result of conversion of colourless catechins to coloured polyphenols including theaflavins (TFs) and thearubigens (TRs). The product is then dried in a fluidized bed in a firing stage, resulting in production of black leaf tea which is sorted and graded. The black leaf tea may then be used in conventional manner, by infusion in water (usually hot) to produce a red-brown coloured beverage.
The black leaf tea may alternatively be further processed to produce tea powders with specific desired properties such as solubility in cold water, clarity, colour and taste. A typical use of such powder is as an ingredient in an instant tea powder mix. Alternatively, the tea powder may be used to produce tea based soft drink beverages such as those sold under the Trade Mark LIPTONICE.
As a further possibility, tea powder may be produced from green tea leaf by a slurry fermentation process. Green tea leaf is macerated by passage through sets of CTC rollers and then slurried in water to produce a dilute slurry of macerated tea leaf. The slurry is fermented at a temperature of about 25.degree. C. by bubbling air or oxygen through the slurry for a suitable time. The resulting product undergoes a deleafing process in which insoluble material is removed and discarded, leaving a pale coloured solution of tea solids. The solution is concentrated, polished and freeze or spray dried to produce water soluble tea powder.
An important consideration in tea processing is the colour of the ultimate tea or tea-based beverage to be drunk by a consumer, it being desirable for the beverage to be clear and have a bright red-yellow colour so that the beverage looks attractive.
The present inventors have surprisingly discovered that use of zeolites in the manufacture of tea products has a desirable effect on the colour of tea beverages produced therefrom.
Zeolites are a large family of aluminosilicate minerals (both natural and synthetic) having a negatively charged framework structure with cavities housing cations typically of one or more alkali or alkaline earth metals such as sodium, potassium, magnesium, calcium and barium. The general formula of zeolites is: EQU M.sub.x (AlO.sub.2).sub.x (SiO.sub.2).sub.y !.sub.n.wH.sub.2 O
where:
n=valence of cation or cations M PA1 w=number of water molecules per unit cell PA1 x and y=total number of tetrahedra per cell PA1 Zeolite 3A=K form (less voidages, 3.ANG. diameter cages) PA1 Zeolite 5A=Ca form (fewer cages due to divalent cation, 5.ANG. diameter cages). PA1 CG180 (from Crosfield) is a sodium analogue of zeolite 3A, having sodium ions in 3A diameter cages.
It is known to use zeolites, particularly zeolite Y, for decaffeination purposes. See, for example, EP 0013451 (Union Carbide), EP 0042294 (General Foods) and CA 2047500 (Jacobs Suchard).
SU 1342470 (Georgian Institute of Subtropical Industries) describes drying of tea leaves for black tea production in which drying air is first passed through a layer of zeolite before being passed up through a layer of green leaves, with the aim of reducing the humidity of the air to shorten drying times. Subsequent processing is conventional. The zeolite does not contact the leaves, and has no direct involvement in tea processing reactions.
JP 63-24850 (Soft Silica KK) describes use of zeolite, particularly soft porous silicate China clay, in processing of green tea with the aim of removing contaminating ions. Zeolite powder or supernatant liquid obtained by mixing zeolite with water is added to fresh leaf prior to or during the steaming stage of production of green tea. This results in production of tea with improved aroma, taste and colour. Green tea production does not involve production of the red/brown colours appropriate to black tea products.
It is also known from JP 04-330908 (Shinozaki) to incorporate calcined zeolite in filter paper, with the aim of eliminating filter paper odour and dissolved filter paper microparticles. Such zeolite-containing filter paper may be used as material for a tea bag containing black tea.
UK patent specification GB 1,382,896 (The Procter & Gamble Company) discloses a method of carbonating aqueous beverages that involves using crystalline aluminosilicates containing gaseous carbon dioxide as a "molecular siever".
However the present invenors are not aware of any prior disclosure of use of zeolites in the manufacture of tea products to generate red colour species.